This invention concerns powered hoists which are used for lifting loads by winding up a wrappable element such as chain or cable, with the load attached to a fitting at the lower end of the wrappable element. A chain is wrapped partially around a sprocket which is rotated to raise the load, and link chain is collected during a rising of the load chain by being directed into a receptable. A cable element can simply be wound up on a drum rotated to raise or lower the load. Such hoists are in widespread use, particularly in production in factories where hoists are in constant use, particularly the so called “air balancer” hoists are controlled and powered to neutralize the load and create the effect of weightlessness of the load in order to be easily maneuvered by production personnel.
Such extensive use of hoists may cause fatigue weakening of the chain or cable load carrying element, particularly if proper lubrication is not provided. The section of a chain which is repeatedly wrapped on the sprocket also wears due to the repeated wrapping and unwrapping and may thereby weakened appreciably over time. Either factor may lead to sudden breakage of the chain if it is not replaced periodically before significant weakening occurs as a part of a careful maintenance regimen. Since such breakages could potentially allow a load to be dropped, damaging equipment and/or injuring personnel, it would be highly desirable to prevent a load from ever dropping in the event a load carrying element such as a chain (or cable) to the load suddenly breaks.
There has heretofore been a number of occurrences of such breakages, and in recognition of that fact, various measures have been devised in an attempt to protect against the possibility of damage or injuries when a chain or cable breaks.
In the context of a hoist used to lift a platform supported by two chains or cables each at a respective end of the platform, there has heretofore been proposed to provide each of the chains with a slack sensor determining when a slack develops in one of the chain or cable supports which occurs when the other of the support chains or cables breaks.
When this occurs, a brake is automatically applied to the unbroken cable on the other side to keep the platform from falling. Such an arrangement is described in U.S. Pat. No. 6,942,070.
Another hoist safety arrangement is available commercially which comprises a load arrester which has a separate cable or chain support connected to the load. The main cable/chain is connected to the upper side of the arrester and suspended from the hoist for up and down movement. This device senses when an excessive downward acceleration of the load occurs. The device then activates a brake to hold separate load arrester housing.
This approach is relatively complex and allows the load to drop for a significant distance before activation of the load arrester brake occurs. This allows significant momentum to build up which increases the forces necessary to be applied to stop the falling load, which increased force could possibly break the separate cable, and would at very least create a sharp jerking of the load.
It is an object of the present invention to provide a simple and reliable safety arrangement for a hoist which prevents a dangerous dropping of a load when a supporting chain or cable element breaks.
It is a further object to provide such security arrangement which quickly acts to hold a load suddenly released by a chain or cable breaking so as to not allow significant continued free fall motion of the released load to thereby minimize the momentum developed by the load in the event of a breakage of the main support chain or cable.